1. Field of the Invention
The present invention relates to an x-ray optical system. More particularly, the present invention relates to an x-ray optical system which conditions an x-ray beam.
2. Description of Related Art
X-ray optics are used in many applications such as x-ray diffraction analysis and spectroscopy that require the directing, focusing, collimation, or monochromatizing of x-ray energy from an x-ray source. Researchers have long employed focusing x-ray optics in x-ray diffraction experiments to increase the flux incident on the sample and hence to increase the signal to noise ratio of radiation received by a detector.
A focusing optic increases the flux by directing a large number of photons through the sample. However, for focusing multilayer optics, the convergence angle of such optics limits their applicability in many applications, since for an application, a different convergence angle, and thus a different optic, is often needed for different types of samples. Moreover, a number of optics with different focal lengths are used to accommodate the needs of different applications. Hence, a different focusing optic is often used for the same measurement of different samples, or for different measurements of the same sample. Using different optics is inefficient and uneconomical since changing the optical elements is a costly and time consuming drain on researchers, in particular, and industry, in general.
Optics with adjustable focal distances have been proposed. An example of such an optic is a traditional bending total reflection mirror. However, the alignment and adjustment of these mirrors are very time consuming and difficult to perform, and any imperfection in the alignment or adjustment of the optic degrades the system performance. Moreover, this approach is not generally applicable to multilayer optics, because of the inability of the variable bent multilayer mirrors to satisfy both the Bragg condition and geometric condition, which have to be satisfied simultaneously.
The reflective surfaces in the present invention are configured as multilayer or graded-d multilayer x-ray reflective surfaces. Multilayer structures only reflect x-ray radiation when Bragg's equation is satisfied:nλ=2d sin θ,  (1)where
n=the order of reflection
λ=wavelength of the incident radiation
d=layer−set spacing of a Bragg structure or the lattice spacing of a crystal
θ=angle of incidence
Multilayer or graded-d multilayer reflectors/mirrors are optics which utilize their inherent structure to reflect narrow band or monochromatic x-ray radiation. Multilayer structures generally comprise light element layers of relatively low electron density alternating with heavy element layers of relatively high electron density, both of which define the d-spacing of the multilayer. The bandwidth of the reflected x-ray radiation can be customized by manipulating the optical and multilayer parameters of the reflector. The d-spacing may be changed depthwise to control the bandpass of the multilayer mirror. The d-spacing of a multilayer mirror can also be tailored through lateral grading in such a way that the Bragg condition is satisfied at every point on a flat or curved multilayer reflector.
Curved multilayer reflectors, including parabolic, elliptical, and other aspherically shaped reflectors must satisfy Bragg's law to reflect a certain specific x-ray wavelength (also referred to in terms of energy or frequency). Bragg's law must be satisfied at every point on a curvature for a defined contour of such a reflecting mirror in order to maximize the intensity of the reflected x-ray radiation. Different reflecting surfaces require different d-spacing to reflect a specific x-ray wavelength. This means the d-spacing should be matched with the curvature of a reflector to satisfy Bragg's law such that the desired x-ray wavelength will be reflected. Since Bragg's law must be satisfied, the incident angle and d-spacing are normally fixed and thus the reflected or working wavelength is fixed.
Multilayer optics having an adjustable working wavelength are disclosed by Jiang et al. in U.S. Pat. No. 6,421,417, the entirety of which is hereby incorporated by reference. In one example, the wavelength of an optic may be adjusted by changing the curvature of the optic. In this case, a change in the curvature requires realignment of the optic. Therefore, such optics cannot be fully optimized for the best performance at the different wavelengths.
In single crystal diffraction, for example, monochromatic x-rays, usually of a single wavelength, produce a pattern of reflections which provides information about the crystal under analysis. More information about the crystal can be collected if x-rays of more than one wavelength are used. Accordingly, a multiple corner Kirkpatrick-Baez type x-ray optic assembly including at least two corners, each of which is configured to reflect a predetermined wavelength, may be used to analyze a sample. A single corner Kirkpatrick-Baez beam conditioning optic assembly is disclosed by Gutman et al. in U.S. Pat. No. 6,041,099, and a multiple corner Kirkpatrick-Baez beam conditioning optic assembly is disclosed by Gutman et al. in U.S. Pat. No. 6,014,423, the entireties of which are hereby incorporated by reference herein.
In a multiple corner optic assembly, for example, one corner may be configured to reflect x-rays of a first wavelength and another corner may be configured to reflect x-rays of a second x-ray wavelength. If an x-ray source with a particular target material selected to operate at the first wavelength irradiates the multiple corner optic assembly, the corner configured to reflect x-rays of the first wavelength will reflect radiation of the target characteristic emission line, or monochromatic radiation, and the second corner configured to reflect x-rays of the second wavelength will collect and reflect some portion of continuous radiation with a wavelength around its characteristic emission line, thus creating unwanted background radiation.
Thus, there is a need for a more efficient x-ray system which provides flexible optical solutions. In particular, there is a need to reduce the background radiation produced in x-ray systems comprising optic assemblies configured to reflect x-rays of more than one wavelength. Further, there is a need for an x-ray system capable of adjusting beam convergence or focal spot size in an x-ray system comprising a multiple corner optic assembly.